This invention relates to circuit-protecting fuses, and more particularly, to circuit-protecting fuses of the so-called secondary voltage type, wherein the fuse is intended for use in circuits carrying voltages of up to about 600 volts. Fuses designed for use in this voltage range, and those with which the present invention is concerned, are sealed such as to prevent escape therefrom of flames or ionized gases and the like.
There are many different classifications of fuses for use in the aforesaid secondary voltage range, and typically the fuses are designed to operate at up to 250 volts a-c or up to 600 volts a-c, over a span of 6 current-carrying ratings, as for example, from 1 to 30 amperes, from 31 to 60 amperes, from 61 to 100 amperes, from 101 to 200 amperes, from 201 to 400 amperes, and from 401 to 600 amperes. The fuses within each of these ampere ratings have different size casings or housings than in the other ratings, and the fuses in the respective ratings must meet several different design criteria. Some of the design standards which must be met in order for the fuses to obtain UL approval are peak let-through current (Ip), or in other words, maximum instantaneous current through the fuse during the clearing time and up to the time of melting, or time elapsing from the beginning of an overcurrent condition to the final circuit interruption, and the total energy available as a result of current flow, expressed as I.sup.2 t and designated as Clearing I.sup.2 t or total I.sup.2 t. There are many other design parameters, as set forth in Underwriters' Laboratories, Inc. Bulletin, UL 198.4 of May 30, 1973. However, the above-noted parameters or standards are the major conditions imposed on obtaining UL approval. The above-noted bulletin, UL 198.4, gives the maximum clearing times, peak let-through current and Clearing I.sup.2 t for a Class R fuse at the various ampere ratings from 0 up to 600 amperes. As noted in the said bulletin, Class R fuses are of the nonrenewable cartridge type and have an interrupting rating of 200,000 rms symmetrical amperes. The maximum clearing time for a Class R fuse at 200% rating ranges from 2 minutes to 12 minutes for the various ampere ratings from 0 to 600, and the maximum acceptable peak let-through current (Ip) and total I.sup.2 t at a short circuit current of 200,000 amperes, ranges from 14,000 amperes and 50,000 units, respectively, up to 100,000 amperes and 12 million units, respectively, at a cartridge size rated for 600 amperes.
Thus, as can be seen, large amounts of energy are involved.
Electrical fuses act as a safety valve in the event of a sustained overload or fault condition or short circuit current in an electrical circuit, and the fuse is designed to open or clear the excessive current safely and without damage to equipment or circuit components or personnel. However, as noted previously, unlike a safety valve, the built-up energy must be contained within the fuse and rupture or venting of the fuse during clearing is to be avoided, such that flames or ionized gases and the like do not escape therefrom and thus create other potentially dangerous situations.
One of the most difficult problems to be overcome in designing a fuse meeting the above conditions is the fact that at the high energy levels encountered, when a fault condition occurs and the fusible portion of the fuse melts and vaporizes, there is a tendency for the electrical energy to form an arc and jump across the vaporized section, thus failing to interrupt the fault condition current, and consequently resulting in damage to other circuit components or to expensive equipment or to personnel and the like.
Various attempts have been made in the prior art to solve this problem, and such attempts have ranged from the provision of fusible links made of silver encased or embedded in silica sand, to fuses having copper or copper alloy fusible links and provided with as many as 200 separate current-conducting paths or arcing paths, whereby the strength of the current flow through each arcing path is proportionately reduced to prevent arcing occurring at overload conditions. The silver fuses, although very effective due to the abrupt melting point of silver and the fact that vaporized silver is not an electrical conductor, are relatively expensive, and thus are suitable for use only in very critical applications where cost is not of especial concern. The copper and copper alloy fuses, on the other hand, which include a large number of current paths or arcing points, are also expensive and difficult to manufacture, due to the complicated fabrication techniques of producing the fusible links, and also some such fuses are relatively fragile and difficult to handle during assembly. Both of the above-described types of fuses have the fusible links thereof embedded in a body of silica sand, and when a fault condition occurs, such that the fusible link melts and vaporizes, the temperature produced by melting and vaporizing of the silver link thereby actually fuses or melts an adjacent portion of the sand, forming a type of glass known as fulgurite, which acts as an insulating barrier between the adjacent portions of the fusible link on opposite sides of the vaporized section, to prevent arcing thereacross. However, notwithstanding the large number of arcing paths provided in prior art copper or copper alloy fuses, it has been observed that prolonged arcing does occur such that the surrounding silica sand actually forms a hollow shell rather than a solid plug of fused glass-like material to block the arc.
A common fuse design of the type with which the present invention is concerned is a so-called dual element fuse, in which a time delay section is provided between spaced short circuit sections, whereby under prolonged low overload conditions the time delay section is designed to gradually build up heat and eventually interrupt the circuit, and under severe overload conditions or short circuit conditions, the fusible links at opposite sides of the time delay section are designed to melt and vaporize to interrupt the circuit. In this type fuse, fiber washers or partitions are provided in the barrel of the fuse between the short circuit sections and the time delay section to prevent or restrict entry of the silica sand filler into the time delay section where it might interfere with proper operation of the time delay section. However, analysis of prior art fuses tested under severe overload conditions indicates that the pressure developed when the fusible link or links vaporize actually blows the fiber washers into the time delay section and permits displacement of the silica sand from the short circuit sections, thus reducing the efficiency of the silica sand in forming a barrier to the arc generated at the short circuit section. Consequently, the fuse fails to perform according to the design considerations necessary for that fuse.
All of the above problems are effectively and economically solved by the present invention. For example, in accordance with the present invention both one-time fuses employing zinc links or links of copper or copper bearing metals, and dual-element fuses, are provided with means at the short circuit sections made of a material which generates an arc quenching gas when heated to a predetermined degree, whereby the heat generated as a result of a fault condition causes production of a deionizing gas from the means to thus extinguish any arc which may form at the vaporized section of the fusible link. Additionally, because of the use of the material with its arc-extinguishing characteristics, a significantly smaller number of current conducting paths or short circuit paths are required than are required in prior art devices, with the result that upon the occurrence of a fault condition, faster heating of the arcing points and faster clearing time for the fuse is obtained than with prior art fuses. For example, a fuse at a particular rating may require only 4 short circuit paths in accordance with the present invention, whereas a prior art fuse of the same rating may require up to 200 short circuit paths. Additionally, in accordance with the present invention, an adhesive cement is provided at the peripheries of the fiber washers, separating the short circuit sections from the time delay section, whereby upon a fault condition occurring, the washers are not deformed or deflected into the time delay section, and the silica sand filler is thus maintained in the short circuit sections, and any ionized gases or deionizing gases produced from the arc quenching means are also maintained in the short circuit sections, whereby any arcs tending to form are quickly extinguished. The adhesive cement is also provided at the end caps and blades of the fuses to effect a secure and pressure tight seal, to thus prevent escape of ionized gases or flames and the like from the fuse under overload conditions. Further, in accordance with one form of the invention, the fiber washers are replaced with washers made of an arc quenching material, whereby the tendency of any arc to form in the time delay section when the time delay section interrupts the circuit under conditions of prolonged overload is prevented by the release of deionizing gases from the arc quenching material.
A preferred arc quenching material to extinguish the formation of arcs in the fuses according to the present invention is an acetal resin plastic material, because of its exceptional non-tracking and non-carbonizing characteristics, and also because it has good electrical and insulating properties not affected by changes in environment. Also, the cement is preferably an inorganic silicate adhesive.